Ion-scale Flux Ropes Research Articles

Solar Orbiter observations of an ion-scale flux rope confined to a bifurcated solar wind current sheet

Context.Flux ropes in the solar wind are a key element of heliospheric dynamics and particle acceleration. When associated with current sheets, the primary formation mechanism is magnetic reconnection and flux ropes in current sheets are commonly used as tracers of the reconnection process.Aims.Whilst flux ropes associated with reconnecting current sheets in the solar wind have been reported, their occurrence, size distribution, and lifetime are not well understood.Methods.Here we present and analyse new Solar Orbiter magnetic field data reporting novel observations of a flux rope confined to a bifurcated current sheet in the solar wind. Comparative data and large-scale context is provided by Wind.Results.The Solar Orbiter observations reveal that the flux rope, which does not span the current sheet, is of ion scale, and in a reconnection formation scenario, existed for a prolonged period of time as it was carried out in the reconnection exhaust. Wind is also found to have observed clear signatures of reconnection at what may be the same current sheet, thus demonstrating that reconnection signatures can be found separated by as much as ∼2000 Earth radii, or 0.08 au.Conclusions.The Solar Orbiter observations provide new insight into the hierarchy of scales on which flux ropes can form, and show that they exist down to the ion scale in the solar wind. The context provided by Wind extends the spatial scale over which reconnection signatures have been found at solar wind current sheets. The data suggest the local orientations of the current sheet at Solar Orbiter and Wind are rotated relative to each other, unlike reconnection observed at smaller separations; the implications of this are discussed with reference to patchy vs. continuous reconnection scenarios.

Particle-in-cell Simulations of Secondary Magnetic Islands: Ion-scale Flux Ropes and Plasmoids

Abstract Ion-scale flux ropes and plasmoids are secondary magnetic islands produced during magnetic reconnection in various heliospheric plasma environments. Here we study the structure of secondary islands and the particle dynamics within them using particle-in-cell simulations. Ion-scale flux ropes (secondary islands with a strong core field) are formed in a strong guide field regime, whereas ion-scale plasmoids (secondary islands with a weak core field) are formed in a weak guide field regime. Currents in both types of secondary islands are carried primarily by electrons. Both types of secondary islands have a magnetic tension force pointing radially inward toward their center. In the flux rope type, this inward tension force is balanced by an outward magnetic pressure gradient of the strong core field; in the plasmoid type, it is countered by an outward thermal pressure gradient caused by electron and ion energizations. The transition between these two types occurs when B g /B 0 = 0.1–0.2 (B g is the guide field, and B 0 is the asymptotic magnetic field).

Observations of Small-scale Magnetic Reconnections within a Magnetic Flux Rope in Earth’s Magnetosheath

Abstract The small-scale magnetic reconnection, which has been observed in turbulent plasma, is detected in the interior regions of an ion-scale flux rope in the magnetosheath. Reconnecting current sheets are configured with a nearly symmetric inflow boundary condition and a large guide field of six times the asymptotic field. The evidence of ongoing reconnection is consistent with the standard reconnection model, except no ion flow is detected. In this study, the electron shear flow near the current sheets allows the reconnections to occur. Strong dissipation with these reconnections indicate that electromagnetic energy can be effectively transformed into electron heating and kinetic energy. In particular, the small-scale reconnections do not result in an increase in high-energy electrons, but are responsible for an increase in electron flux over a lower energy range of 100–200 eV. Observations indicate that small-scale reconnection is very common and provides an important channel for energy dissipation in the magnetosheath plasma.

Internal structures of the ion-scale flux rope associated with dayside magnetopause reconnection

Ion-scale magnetic flux ropes have been observed in the reconnection diffusion region. In this paper, we show a typical ion-scale flux rope observed by the Magnetospheric Multiscale spacecraft at the subsolar magnetopause. The most intriguing feature of this flux rope is that the rapid variations of electron density are correlated with the changes of electron temperature and pitch angle distribution inside the flux rope. In the region of low electron density, the perpendicular temperature is higher than parallel temperature. Furthermore, the electrons within higher energies (>70 eV) are mainly trapped near pitch angle $\sim 90^{\circ}$, but electrons with lower energies (<70 eV) are missing. However, in the region of high electron density, perpendicular temperature is lower than the parallel temperature. In addition, the pitch angle of electrons with higher energies is essentially field-aligned, and the fluxes of electrons with lower energies rapidly increase. It implies that the coexistence of different magnetic field line topologies inside the ion-scale flux rope is responsible for the observed electron pitch angle distributions within the flux rope. These results can be useful for understanding the reconnection process in three-dimension and the generation mechanism of the ion-scale flux rope.

Formation of Macroscale Flux Transfer Events at Mercury

Abstract Flux transfer events (FTEs) are magnetic flux ropes that are produced via magnetic reconnection at the planetary magnetopause where the solar wind directly interacts with the magnetosphere. Previous observations show that FTEs with a duration of several seconds, corresponding to a spatial scale of ∼0.5–1 R M, can occur at Mercury. However, the formation of these macroscale FTEs at a small dimensional magnetopause with a radius of ∼1.5 R M remains unclear. Here, we report the observations of active magnetic reconnection events at Mercury’s magnetopause by the MESSENGER spacecraft. The reconnection process is dominated by the formation of a series of multi-scale FTEs. Ion-scale flux ropes, typically with durations of ∼1 s or less, may be produced by the tearing instability in the thin current sheet near the subsolar position. Moreover, the commonly observed macroscale FTEs consist of three to tens of successive small-scale FTEs. We propose that macroscale FTEs at Mercury are generated by the interaction and merging of multiple ion-scale flux ropes, probably through two or more steps. This is distinct from the formation of typical FTEs, mainly between a pair of X-lines, at Earth’s magnetopause. Thus, the formation and evolution of FTEs may differ among planetary magnetospheres with a vast range of scale sizes. We further conclude that Mercury’s magnetopause is a natural plasma laboratory to study flux rope dynamics and evolution for the upcoming Bepi-Colombo mission.

A three-dimensional model of spiral null pair to form ion-scale flux ropes in magnetic reconnection region observed by Cluster

The magnetic structure and topology of the three-dimensional magnetic reconnection region are significantly dynamic and complex. Small-scale flux ropes and magnetic null points are frequently detected in the reconnection outflow region and diffusion region due to the increased in situ measurements at high temporal cadences. Previous studies have demonstrated that X-line and small-scale flux ropes are both related to null points. In this study, by applying a fitting-reconstruction method with the input of the Cluster dataset, we reveal three types of spiral null pairs that serve as the skeleton of the flux ropes. Two spiral nulls can be connected by a spine, or by a separator, or by both a spine and a separator. A theoretical model is proposed to explain these spiral null pairs. The observational results and the model indicate that the number of magnetic loops of the flux rope is restricted by the linkage pattern of two nulls, while the flux rope is confined by the two nulls and their fan surfaces. The model predicts that the magnetic perturbations in the reconnection region can transform the linkage types of the nulls and eventually lead to the evolution of flux ropes.

Observations of whistler waves in two sequential flux ropes at the magnetopause

Two ion-scale flux ropes (FRs) embedded in southward flow at the dayside magnetopause were encountered by Magnetosphere Multiscale (MMS) spacecraft sequentially in 9 s. Super-Alfvenic electron jet, non-zero electric field in electron frame, and strong current are observed in these two FRs. Specifically, whistler waves are observed inside and outside the FRs. The properties of whistler waves inside the FRs are quite different from whistler waves outside the FRs. Whistler waves outside the FRs have a frequency range between 0.1$f _{\mathit{ce}}$ and $f _{\mathit{ce}}$ ($f _{\mathit{ce}}$ is the electron gyro-frequency) and they propagate parallel to the magnetic field. Whistler waves inside the FRs have a frequency range between 0.5$f _{\mathit{ce}}$ to $f _{\mathit{ce}}$ and they propagate antiparallel to the magnetic field. The intensity of whistler waves outside FRs is stronger than the whistler waves inside FRs. Electron beam instability may be the reason for the generation of whistler waves observed here. The difference between whistlers inside and outside FRs may have a close relationship with different magnetic topology and electron distribution inside and outside FRs.

Effect of Kinetic Alfvén Waves on Electron Transport in an Ion-Scale Flux Rope**Supported by the National Natural Science Foundation of China under Grant Nos 41474145, 41574159, 41731070 and 41504114, the Frontier Science Foundation of the Chinese Academy of Sciences under Grant No QYZDJ-SSW-JSC028, the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDA15052500, and the

At the Earth’s magnetopause, the electron transport due to kinetic Alfvén waves (KAWs) is investigated in an ion-scale flux rope by the Magnetospheric Multiscale mission. Clear electron dropout around 90° pitch angle is observed throughout the flux rope, where intense KAWs are identified. The KAWs can effectively trap electrons by the wave parallel electric field and the magnetic mirror force, allowing electrons to undergo Landau resonance and be transported into more field-aligned directions. The pitch angle range for the trapped electrons is estimated from the wave analysis, which is in good agreement with direct pitch angle measurements of the electron distributions. The newly formed beam-like electron distribution is unstable and excites whistler waves, as revealed in the observations. We suggest that KAWs could be responsible for the plasma depletion inside a flux rope by this transport process, and thus be responsible for the formation of a typical flux rope.

Magnetic depression and electron transport in an ion-scale flux rope associated with Kelvin–Helmholtz waves

Abstract. We report an ion-scale magnetic flux rope (the size of the flux rope is ∼ 8.5 ion inertial lengths) at the trailing edge of Kelvin–Helmholtz (KH) waves observed by the Magnetospheric Multiscale (MMS) mission on 27 September 2016, which is likely generated by multiple X-line reconnection. The currents of this flux rope are highly filamentary: in the central flux rope, the current flows are mainly parallel to the magnetic field, supporting a local magnetic field increase at about 7 nT, while at the edges the current filaments are predominantly along the antiparallel direction, which induce an opposing field that causes a significant magnetic depression along the axis direction (&gt; 20 nT), meaning the overall magnetic field of this flux rope is depressed compared to the ambient magnetic field. Thus, this flux rope, accompanied by the plasma thermal pressure enhancement in the center, is referred to as a crater type. Intense lower hybrid drift waves (LHDWs) are found at the magnetospheric edge of the flux rope, and the wave potential is estimated to be ∼ 17 % of the electron temperature. Though LHDWs may be stabilized by the mechanism of electron resonance broadening, these waves could still effectively enable diffusive electron transports in the cross-field direction, corresponding to a local density dip. This indicates LHDWs could play important roles in the evolution of crater flux ropes. Keywords. Magnetospheric physics (magnetopause, cusp, and boundary layers; solar wind–magnetosphere interactions)

Observations of Slow Electron Holes at a Magnetic Reconnection Site

We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to f(pe) are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.